Power amplifiers are used in communications satellites to amplify RF signals. Such power amplifiers are known to exhibit nonlinearity, whereby the gain and phase of the amplified output RF signal varies nonlinearly with the input signal power. To compensate for this, gain and phase distortions can be applied to the input signal before it reaches the amplifier stage, so as to maintain a constant gain and phase of the output signal.
FIG. 1 illustrates an open-loop control system used in a conventional communications satellite to control the gain and phase of the input signal. The system comprises a power amplifier 101, a variable attenuator 102, a phase shifter 103, an input coupler 104, a detector 105, a processor 106 and a memory 107. An initial calibration procedure is carried out in which the response of the amplifier 101 is characterised over a range of different signal powers. A lookup table (LUT) is created to record suitable gain and phase predistortions to be applied to the input signal for any given power level. The LUT is stored in the memory 107.
During operation, the detector 104 measures the power level of a coupled input signal received from the input coupler 103. The processor 106 then determines the power of the input signal RFIN based on a known coupling factor of the input coupler 103. The processor 106 searches the LUT to determine how the gain and phase of the input signal should be adjusted, and controls the variable attenuator 102 and phase shifter 103 to apply the appropriate gain and phase predistortions. However, a drawback of this approach is the lengthy calibration procedure, which can take up to 48 hrs.